Conversion of synthesis gas to dimethyl ether requires three steps. Conventionally, synthesis gas is produced by reforming hydrocarbon or gasifying a carbon source such as coal or coke. Since this latter synthesis gas usually is too rich in CO to be used directly for dimethyl ether synthesis, an intermediate step is needed for conventional dimethyl ether manufacture. Consequently, the first step in the dimethyl ether synthesis is to adjust the composition of the synthesis gas via the water-gas shift reaction: EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 1)
After the ratio of hydrogen to carbon oxides has been adjusted, the gas is reacted to produce methanol (MeOH): EQU CO+2H.sub.2 .revreaction.CH.sub.3 OH (2)
Finally, methanol is dehydrated to form dimethyl ether (DME): EQU 2(CH.sub.3 OH).revreaction.CH.sub.3 OCH.sub.3 +H.sub.2 O (3)
Reactions (1), (2), and (3) are equilibrium limited and exothermic. Moreover, the catalysts for both methanol synthesis and shift reactions are subject to severe deactivation when overheated. To avoid thermodynamic limitations and excessive catalyst deactivation, conventional gas phase reactors must be run at low per-pass conversions to maintain reactor temperature. Consequently, overall conversion of carbon monoxide to dimethyl ether is limited.
Multi-step processes, which use separate reactors for each reaction, cannot exploit the potential synergism of the three reactions. If these three reactions could be conducted simultaneously, methanol synthesis would drive the forward shift reaction, and dimethyl ether synthesis would drive both the methanol and shift reactions. Consequently, a one-step process is more flexible and can operate under a wider range of conditions than a multi-step process. In addition, multi-step processes require separate reactors, heat exchangers, and associated equipment for each reaction.
A single-step gas phase process would generally require less equipment than multi-step gas processes. However, a single-step gas-phase process would still suffer from a large reactor exotherm due to the high net heat of reaction. Hence, low per-pass conversions would be required to maintain reactor temperature to avoid a short catalyst life due to the large temperature rises associated with these reactions. Since the gas phase reactor is not isothermal, there are often severe equilibrium limitations in reactant conversions per pass.
Much of the prior art for dimethyl ether synthesis focuses on processes using improved catalysts to run shifted syngas (H.sub.2 /CO greater than or equal to 1). Examples include U.S. Pat. Nos. 4,417,000; 4,423,155; 4,520,216; 4,590,176; and 4,521,540. These processes all run in the gas phase, and may be considered multi-step processes in that they all require the feed be shifted via Reaction (1).
Single-step gas-phase processes have been disclosed by Mobil Corp. and Haldor-Topsoe. For example, U.S. Pat. No. 4,011,275 assigned to Mobil Corp. discloses a gas-phase process for coproduction of methanol and dimethyl ether with H.sub.2 deficient syngas feeds. Although there are no examples in the patent, the process is claimed to be useful for improving conversion of synthesis gas. U.S. Pat. No. 4,341,069 discloses a gas-phase process for dimethyl ether production to be used in conjunction with an integrated gasification combined cycle power plant. Examples in the patent show that the catalyst requires frequent regeneration, in some cases on a daily basis. Another gas-phase process is described in U.S. Pat. No. 4,481,305, however, this process is restricted to operation within a narrow range of CO/CO.sub.2 ratio in the feed gas. It should be noted that efficient heat removal to maintain reactor temperature is generally not discussed in these patents. Fujimoto et al discusses in Chem. Letters, p.2051 (1984) the chemistry of the gas-phase one-step processes.
Combined methanol/dimethyl ether synthesis in the liquid phase has been reported by several workers. Sherwin and Blum, in their paper entitled "Liquid Phase Methanol Interim Report, May 1978", prepared for the Electric Power Research Institute, attempted to modify the liquid phase methanol process for coproduction of dimethyl ether by adding acid catalyst components to the system. They observed only traces of dimethyl ether, and concluded that the attempt was unsuccessful. Daroda, et al, J.C.S. Chem. Comm. p.1101 (1980), reported a broad slate of products for reactions of syngas with Fe in 2-methoxyethanol. However, in their system the solvent appears to act as a reactant, and the catalyst produces many side products. Consequently, neither earlier liquid phase process was economic.
UK Patent Application GB 2 093 365 A discloses the catalyst of above-cited U.S. Pat. No. 4,423,155 suitable for gas-phase synthesis of dimethyl ether, and discloses that such a catalyst may be suspended in a slurry for dimethyl ether synthesis in a liquid phase reactor.
An article by J. J. Lewnard et al entitled "Single-Step Synthesis of Dimethyl Ether in a Slurry Reactor" in Chemical Engineering Science Vol. 45, No. 8, pp. 2735-2741, 1990 describes the synthesis of dimethyl ether and methanol in a liquid phase reactor using synthesis gas feed containing between 20 and 60 vol % carbon monoxide. A mixed catalyst containing between 36 and 54 wt % methanol synthesis catalyst and the remainder methanol dehydration catalyst is disclosed. This synthesis is also described in a paper by T. H. Hsiung et al entitled "Synthesis of Dimethyl Ether from Syngas in a Slurry reactor" presented at the AIChE 1990 National Meeting, San Diego, Aug. 19-22, 1990.